1. Field of the Invention
The present invention relates generally to lattice-mismatched crystal structures, and more particularly to semiconductor devices such as electronic and optical devices that use lattice-mismatched crystal structures.
2. Description of the Related Art
Various problems arise from the use of lattice-mismatched crystal structures in semiconductor devices. The problems originate from the electrical characteristics of a semiconductor film crystal whose lattice constant is different in the direction parallel to the surface of the substrate from that of a substrate crystal on which it is grown (the so-called "lattice-mismatched crystal structure").
Chang et al, "Lattice-Mismatched In.sub.0.53 Ga.sub.0.47 As/In.sub.0.52 Al.sub.0.48 As . . . ", J. Appl. Phys., 67 (7), (1 Apr. 1990), pages 3323-3327, discuss a method of improving electron mobility in the channel of a high electron mobility transistor (HEMT) device having an InAlAs carrier supplying layer/InGaAs channel layer structure. Chang et al report that the electron mobility in the channel is improved by stepwise interposing a buffer layer having a structure comprising InGaAs crystal layers that are different in lattice constant between a GaAs substrate crystal and a film crystal constituting the active layer of the HEMT device, thereby reducing dislocations due to lattice mismatch. In the disclosed process, the active layer has a dislocation density (number of defects per unit area) of approximately 10.sup.6 cm.sup.-2. The electron mobility is 8150 cm.sup.2 at room temperature when the total buffer layer is 2.5 .mu.m thick.
Win et al, "Metamorphic In.sub.0.3 Ga.sub.0.7 As/In.sub.0.29 Al.sub.0.71 As layer on GaAs: . . . ", Appl. Phys. Lett., 61 (8) (24 Aug. 1992), pages 922-924, disclose another technique of improving the electron mobility in an HEMT device by reducing the dislocation due to lattice mismatch. Win et al form a single InGaAs graded buffer layer whose lattice constant varies continuously, or a buffer layer having a structure in which an InGaAs graded layer is sandwiched between layers whose lattice constants are constant, between a GaAs substrate crystal and an InAlAs film crystal constituting the active layer of an etched HEMT device. The degree of mismatch is low, as shown by the In composition of 0.3. The electron mobility is 8500 cm.sup.2 /Vs at room temperature for a buffer layer thickness of 1 .mu.m.
In the devices resulting from the fabrication techniques disclosed in each of these articles, only HEMT crystals that are lattice-matched on the GaAs substrate are suitably utilized, and their electron mobilities are insufficient.